1. Field of the Invention
The present invention relates to an internal combustion engine, such as a diesel engine, of a type in which fuel is injected into the cylinders, and more particularly relates to an exhaust gas recirculation system therefor.
2. Description of the Prior Art
An exhaust gas recirculation system for an internal combustion engine is provided with an exhaust gas recirculation (referred to hereinafter as EGR) passage which communicates between the exhaust passage and the intake passage of the engine. A differential in presure occurring between the exhaust passage and the intake passage recirculates part of the exhaust gas to the intake passage. This pressure differential is usually produced by a throttle valve provided at a point in the intake passage. The opening amount of the throttle valve is changed according to the operating state of the engine, and this changes the intake vacuum to which the EGR passage opens, thereby producing a pressure differential along the EGR passage which controls the amount of EGR.
Since the throttle valve provided in the intake system controls the amount of EGR in such a prior art system, however, the flow of intake air is necessarily always somewhat throttled. This impairs the advantage that the puming loss is small in an engine in which fuel is injected. Further, the amount of flow of intake air into the engine is decreased, which deteriorates the burning of fuel within the engine. This may increase the amount of discharged unburnt hydrocarbons and smoke emissions, or decrease the power output of the engine whereby deteriorating proper engine operation. However, since it was absolutely necessary to recirculate exhaust gases, use of the throttle valve could not be avoided.